Telegraph system



Jan. 9, 1940.

F. E. D'HUMY ET AL TELEGRAPH SYSTEM Original Fil edNov. 30, 1935 3 Sheets-Sheet 1 INVENTORS F. E. -d'HUMY BY L.W. FRANKLIN Jan. 9, 1940. F. E. DHUMY ET AL TELEGRAPH SYSTEM Original Filed Nov. 30, 1935 3 Sheets-Sheet 2 INVENTORS FE.dHUMY BY L.W. FRANKLIN 66 RN EY I I m m 0: Itt m m uh l 0O 0% v .55 Ha. I F. nm

F. E. D'HUMY ET AL TELEGRAPH SYSTEM Jan. .9, 1940.

Original Filed Nov. 50, 1935' s Sheets-Sheet 3 llllll C L 1 I D a: i

INVENTORS F.E. dH UMY L.W. FRANKLIN ATT RNEY Patented Jan. 9, 1940 s PATENT OFFICE UNITED STATE TELEGRAPH SYSTEM Original applications November 30, 1935, Serial No. 52,408, and August 13, 1936, Serial No. 95,912. Divided and this application December 5, 1936, Serial No. 114,460

21 Claims. (Cl. 179-4) This invention relates to telegraph systems and more particularly to an improved amplifier for printing telegraph systems or the like employing audible frequency impulses or audible tone signals. The present application is a division of our prior U. S. applications, Serial No. 52,408 filed November 30, 1935, and Serial No. 95,912 filed August 13, 1936.

In printing telegraph, telautograph or facsimile transmission systems for carrying on telegraph correspondence over a standard telephone circuit, telegraph, telautograph or facsimile signals comprising audible frequency impulses or audible tone signals are transmitted over the as telephone circuit. The signals are amplified at the receiving station, rectified and impressed upon the printer or other receiving, apparatus.

The advantage of this system is that telegraph communication may be carried on between any two telephone substations utilizing the regular telephone line circuits. In the operation of the system, however, on account of the varying-attenuation of the line circuits and other causes, the signals are distorted both as to amplitude and duration. In order to operate standard telegraph printing apparatus at a relatively high transmitting speed it is necessary to avoid appreciable distortion of the signals. This isv particularly true of the bias of the signal inasmuch as any change in duration of the impulses forming each code combination reduces the operating margin of the printer'and a comparatively small bias usually .causes the printer to be inoperative or print the wrong characters.

In general terms the object of the present invention is to provide an improved amplifier having such characteristics that the operating limits of the telegraph equipment used in connection with the telephone system permit telegraph com-- 40 munication to be carried on over any circuit over which it is possible to converse.

Another object of the invention is to provide an amplifier for systems of the character referred to above which corrects the shape of the 45 signal impressed upon the receiving apparatus and eliminates or reduces both amplitude and bias distortion of the signal.

Another object of the invention is to provide automatic gain control for an amplifier operating on interrupted or. discontinuous signalling currents as distinguished from a continuous car- A further object of the invention is to provide an improved amplifier for systems of the char- 55 acter described which is arranged to operate a telegraph printer or the like in response to received signals of widely varying amplitude.

Other objects and advantages of the invention will appear from the following description of the preferred embodiments thereof shown on the ac- 5 companying drawings, in which Fig. 1 is a circuit diagram of a printing telegraph system embodying the invention;

Fig. 2 is a similar diagram of a system employing a modified form of amplifier, and 10 Figs. 3 and 4 are graphs illustrating the operation of the amplifiers shown in Figs. land 2.

Referring first to Fig. 1, there is shown a vacuum tube oscillation generating and amplifying unit OI of the inductively coupled feed back 15 type, comprising a vacuum tube VI, feed back coil Ll inductively coupled to a tuned grid circuit including the grid coil L2 and associated tuning condenser C2. A grid condenser 03 and leak resistance R3 may also be included in the grid circuit. The output circuit of the oscillator tube VI is coupled through the condenser CI and resistance Rl to the grid circuit of an amplifying tube V2 and the anode circuit of tube V2is completed through the choke coil L3 to contact I of a telegraph transmitter Ti, the common or bus contact in thereof and by conductor H to battery. The output circuit of the amplifier tube V2 is coupled to a sound producing device or reproducer S through the coupling condenser C5. 30

It will be evident that by opening thecontact l of the telegraph transmitter, the plate voltage of tube V2 is removed, no current will flow through the anode circuit of V2 and consequently no tone will be transmitted to the sound producing device.- 7 Similar oscillators 02, 03, 04 and 05 are provided for the contacts 2, 3, 4 and of the trans- :nitter Tl, the output of each oscillator being similarly coupled to the sound producing device 40 S. Each oscillator is tuned to a diilerent frequency so as to produce its distinct tone in the sound producer S. By closing thecontacts l to 5 of the transmitter T in various combinations, correspondingly combined tones are produced in the sound producer 8.

The transmitter Ti may comprise a series of keyboard controlled contacts but is shown as a tape transmitter such, for instance, as shown in patent to G. R. Benjamin, No. 1,298,440, granted March 25, 1919. The stepping magnet SM of the transmitter is operated periodically by a cam switch l2 driven by a continuously running motor l3. Upon each revolution of the camthepinsofthetspetransmitterarewithdrawn, opening all contacts I to 5, the tape advanced and the pins released to again engage the tape or to pass therethrough whereby to selectively close the contacts I to 5. During closure of the contacts a combination of tones is produced by the speaker S corresponding to the frequency source or sources associated with the closed contacts. Upon withdrawal of the tape pins, the sound producer is silent, and upon repositioning of the transmitter contacts, a different combination of tones is produced. Each combination of tones represents a character code, the character tone signals being of definite length or duration and spaced by intervals of silence.

The signalling currents may be impressed upon the telephone line in any desired manner, as by acoustic coupling or through a coupling transformer. As shown, the sound. producing device 8 is enclosed within a sound deadening container l4 disposed in sound transferring relation to the transmitter T1 of a standard telephone system or circuit for the transmission of voice frequency currents so that the character tone signals are transmitted over the telephone line L and reproduced by the distant telephone receiver TR. The telephone system may comprise a transmission channel of any usual character or series channels of different types and the sending and receiving stations may be connected to the same telephone exchange or different exchanges.

In the receiving means for operation in connection with the transmitting means just described, the tones are picked ,up by a microphone M and applied through a transformer T2 and separate band pass filters BPFI to BPFS to a separate demodulator system and relay tube for each tone frequency used. The operation of'the demodulator is as follows: when a signal is received through the filter BPFI, it is applied to an amplifier tube V3. The signals which have been amplified by tube V3 are applied in pushpull to the grids of tubes V4 and V6 through a transformer T3, which has a center-tapped sec-' ondary. The function of tubes V4 and V5 is to rectify the signals full wave, amplify them and apply the resulting direct current signals to the first pulse magnet PMI of the printer P. These functions are performed in the following manner:

A bleeder circuit is provided from positive battery through resistances R4 and R8 to ground return, or negative battery, as shown. Tubes V4 and V5 obtain their plate voltages from positive battery through the printer magnet PMI. Tubes V4 and V5 have their filament return to the junction of resistors R4 and R5. The grid return from the center-tap of the secondary winding of transformer T3 is made to the grounded end of the bleeder circuit. Therefore, the negative grid bias on tubes V4 and V4 is the voltage drop across the resistance R5; the plate voltage applied to tubes V4 and V5 is the voltage drop across the resistance R4, less the voltage drop through PMI (if any). The value of resistance R5 is such that the plate current, with no signal applied to the grids, is cut off, or nearly so, in tubes V4 and V5. With this plate current interrupted, or nearly so, there is little or no current fiow through the printer magnet PMI.

When a tone is received over the telephone line from the source OI, indicating what is normally a marking signal, this tone is amplified in tube V3 and applied to the grids'of tubes V4 current through the printer magnet PMI.

and V5. On one-half cycle the grid of tube V4 is either positive, or at least less negative, than before and therefore plate current will fiow through this tube and through the printer magnet PMI. on alternate half cycles the grid of tube V5 becomes less negative and also passes small condenser C6 is connected between the paralleled plates of tubes V4 and V5 to the filament or other unipotential point. The value of this condenser is such that the charging of this condenser during the'time that neither tube is passing current through magnet PMI, and the discharging of the condenser during the time that either tube is passing current through magnet PMI, will effect a filtering action which will hold the current through the magnet PMI at an essentially constant value during the period of any pulse without seriously afiecting the general shape of the printer signal.

Similarly, signals originating at the oscillators O2 to 05 will, after transmission over the telephone system, be passed through selective or band pass filters BPF2 to BPFS, and individual amplifiers A1 to A5 where the signals will be amplified and fed to the corresponding printer magnets PMl to PMS of a standard multiplex printer P. A sixth pulse magnet PMS in the common return of each of the magnets PMI to PMS is energized whenever a selecting magnet is operated to effect the printing, as is well known in the art.

In systems of this kind a problem often encountered is the tendency of tails to form at the end of signals. The current impulses received are not cut off sharply and this distortion may be sufiicient to limit the operating speed considerably below that desired. This eifect is quite noticeable in sharply tuned filter circuits, and is also noticeable where there is a mechanical or acoustical link in the circuit, such as the diaphragms of the telephone sets and of the microphones and loud speakers used in this system. When a signal which has been impressed on the system is suddenly stopped, there is a tendency for the diaphragms to keep on-vibrating, the amplitude dying out at a rate dependent upon the damping of the diaphragm. The same inertia effect takes place in a sharply tuned filter, since the damping is low, the oscillating energy being electrical in this case rather than the energy of mechanical motion. The effect of this tailing" can be compensated for manually, by adjusting the biasing eifect somewhere in the system to a value intermediate to the maximum value of the signal and the tail amplitude. However, it is desirable to have means which will automatically compensate for this effect, and such an arrangement is shown in the amplifier of Fig. 1, in the combinations of resistance and capacity shown as RS-CI, and also as RI-Cl.

By referring to Fig. 3, the effect of this talling" may be more easily understood. In curve 3A are shown the signals of uniform length or duration as they are sent to the sound producer S. Curve 3B shows a rather weak signal as it might be received at the distant end of a long circuit, or one with high attenuation. The signalsshownincurvewaresimilartothosein curve 33 except that the amplitude is much higher, such as might be received from a nearby sending station over efficient circuits. For the proper reception of signals as shown in curve 33, the grid bias of tubs V4 and W which may be the fixed voltage across resistance B! would be a,1ae,eee

such that the plate current would be increased in tubes V and V5 whenever the signal amplitude passed above the dotted lines H representing the fixed bias of tubes V4 and V5. However, when a signal oi high amplitude is received, as'shown in curve 30. this bias must be increased to a value shown at y--1/ in order to receive practically unbiased signals because'of the "tailing" of the signals. Referring back to curve 33, it is seen that with the bias set at level 21-11 for reception of loud signals, the receiver would not respond to signals of small amplitude, since the bias at all times would override the signal as shown. Conversely, if the bias were set at the level x-m, the optimum point for weak signals as shown in curve 33, strong signals would be badly under-biased, due to the tailing eiiect and would appear in the output of tubes 'V4 and V5 similar in form to that shown in curve 3D, and if the signal amplitude were much larger the spacing signals would not be received. Since it is desired that this system be operable without special adjustment on any telephone circuit, it becomes necessary to furnish means whereby the bias may be changed to correspond to the strength of the received signals automatically. Also, a fading effect is sometimes experienced on a telephone circuit where the efflciency of the circuit will vary during a call, and automatic means for compensation become a necessity for this reason. At the transmitting speeds for which high-speed transmitters and printers are designed to operate, it is essential to maintain the duration and magnitude of the controlling impulses between definite limits.

Consider the resistance R! and the condenser C9 in the center-tap connection oi the secondary winding of transformer T3. When no signal is being received, there is no current flowing through resistance R! and therefore no voltage drop across this resistance. The initial negative grid bias on the demodulator tubes V4 and V5 is the voltage drop across the resistance R5 and is so adjusted that the demodulator tubes will respond to theweakest signals which will be encountered. On weak signals the grids of tubes V4; and V5 will not be driven positive so no current will flow through the resistance R! and the minimum or initial bias will still be applied to tubes V4 and V5. However, when a strong si nal is received, the grids of tubes V4 and V5 aredriven positive, allowing grid current to flow through resistance R'l, which will create a negative potential with respect to ground on-the grids as a result of the direct current flowing through the resistance R1. This voltage is more or less proportional to the strength of the received signals, so that the bias on tubes V4 and V5 is automatically varied, the bias increasing when 'a signal is received, and approaching the minimum bias set by resistance R5 when no signal is being received. The action of this resistance alone would not be suflicient, as it would not cut off the tails" of the signal, since the rise and fall of the bias would be practically instantaneous. However, by timing this change in' voltage across resistance Rl by means of the condenserCQ. the rise and fall of this added bias across resistance RI may be made slower than the decay of the "tail" of the signal. The eifective bias will be as shown at 2-2 of curve '30 for loud signals, and will drop to the value shown :at for weak signals. Conseis, as practically imbiased impulses. The size of the condenser C8 is such as to give the gain control a time constant of the same order as the shortest signal received.

The resistance R6 and condenser C8 exert a 5 similar control over the amplifying characteristics of tube V3 when the signal input is sufficiently large. For ordinary signals there will be no grid current flow in the input circuit of this tube and thus the grid bias will not be affected.

Obviously, the telegraph transmitting and receiving apparatus may, if desired, be connected directly or through acoupiing transformer to the telephone circuit or line -L instead of being acoustically coupled thereto. However, it is desirable in many instances to avoid modification of the telephone substation equipment or any direct connection to the telephone circuit. Although the use of the acoustic coupling elements S and M introduces the diiiiculty that the diaphragms thereof as well as the diaphragms of the telephone transmitter and receiver tend to continue to vibrate after the end of each signal im= pulse, thereby producing biased signals, the tailing of the signals in this manner may be reduced by picking up the received signals inductively from an element of the telephone system such as the induction coil in the bell box, this modification being illustrated in, Fig. 2.

Referring to this figure of the drawings, the invention is illustrated in connection with station A of a telephone system, said station comprising the usual substationset including a handor equivalent transmission channel. 4

In the embodiment of the invention shown, a

telegraph transmitter T2, for example a startstop keyboard or tape transmitter, is arranged to key the output of a low-frequencyoscillator 06, for example of 750 cycles, to productaudibletone permutation code signals to be transmitted over the telephone circuit to a subscriber connected to station A through the telephone exchange EX. The tone signals are. impressedupon the transmitter of thehandset through the acoustic coupling unit containing a sound reproducer 26 connected to the terminals of the I oscillator 08. 'It will be apparent that in a facsimile system a conventional facsimile trarismitting device is arranged to impress audible signals upon the telephone transmitter in the same manner as the reproducer 26 and keyboard or tape transmitter T2. The term telegraph is therefore intended to cover other signaling equipment than that shown by way of example in the drawings.

'Ih'e tonesignals are transmitted over the line .24 to the receiving station where the. signals arepicked'up by a coil corresponding :to thecoil an in mutual inductive relation to the induction coil 70 23 or equivalent element of the substation set'at the receiving station. The coil 30 may be inside or outside of the bell box and is preferably constructed with a great many turns, for example,

several thousand turns; and with as large an area 15 as practicable. The coil is preferably centertapped or connected to a tapped resistor 3| as indicated. The signalling impulses induced in the coil 30 are amplified by a preamplifier tube V6 having the input circuits thereof arranged in push-pull relation to minimize distortion or interference produced by insufilciently filtered power supply and capacity pickup from the telephone box to the pickup coil of disturbances which may be picked up by the telephone circuit, etc. The coil 30 may be disposed in any position where current will be induced therein by the signalling currents traversing the induction coil 23, although obviously the coupling between said coils is a maximum when the coil 30 is located adjacent one end of the core of the induction coil 23 as shown.

The preamplifier is preferably shielded and mounted close to the coil 30 to minimize extraneous pickup, and the output transformer T4 thereof connected through a cable 33 to an amplifier and inverter unit consisting of amplifier tubes V1 and V8 connected in cascade, an amplifier-rectifier tube V! and an inverter tube V10. Except as noted hereinafter, the amplifier unit is a conventional low-frequency amplifier and need not be described in detail. The tube V1 is preferably a variable-mu tube such as the type 35 tube with the screen grid and plate strapped together. The amplifier gain is controlled by biasing the control electrode of tube V1 partly in accordance with the grid current of said tube and partly in accordance with the grid current of tube V9 by the arrangement of resistors 35, 33 and 31 which may be one-tenth megohm each. Theresistor 35 is connected in the grid return circuit of tube V1, the resistor 36 in the grid return circuit of tube V9 and the resistor 31 between the grid terminals of both of the other resistors.

With this arrangement it will be apparent that on signals of intermediate amplitude, the bias of the control grid of tube V1 is determined partly by the grid current of tube V1 but principally by the larger grid current of tube V3, and the gain in the amplifier is less for signals of average strength than when weak signals are impressed upon the tube V1. On the other hand for stronger signals the gain in both tubes and particularly in the tube V1 is further reduced by the increased negative bias to the control electrode of tube V1 so that the rectified output of tube V9 is substantially constant over a wide I range of amplitude of received signals.

The resistor 38 between the cathode and control grid of the tube V1 is traversed by the cathode-anode current and the potential drop in the resistor biases the grid with respect to the cathode. The grid bias depends upon the resistance of resistor 38, the resistance chosen deing printer P which is shown as a simplex or start-stop printer. The tube VII is normally conductive by reason of the connection of'the control electrode thereof through a resistor 33 to a point of positive potential with respect to the cathode of said tube. The resistor 33 is also in series relation with the anodes of tube V! whereby when signals are impressed upon the latter, the current fiow through the resistor 33 causes the control electrode of the tube VI. to become more negative and interrupts the flow of current to the printer magnet for the duration of each signal. Thus the spacing signals, corresponding to tone signals received over the line, are converted into no-current signals to control the printer P in the usual manner.

The automatic amplifier gain control circuit described above differs from the usual gain control circuit utilized in radio frequency amplifier and detector systems as the latter utilizes a common bias resistor in the grid-return circuits of the detector or rectifier and of the preceding amplifier. The arrangement shown has the advantage over such systems that upon receipt of strong signals, the bias voltage produced by the grid current of the amplifier V1 does not block the tube V9 since the resistors 35, 36 and 31 are so arranged that the grid bias of the tube V3 resulting from the grid current of tube V1 is only a fraction of that impressed upon the control grid of the tube V1. Thus the amplifier is not rendered inoperative by the reception of ab-- normally strong signals and the system is operative over a wider range of signal variation and may be made sensitive to weak signals without blocking when strong signals are received. Obviously, the resistance ofresistors 35, 33 and 31 may be varied to a considerable extent without affecting the operation of the amplifier. The values given are merely illustrative and depend to a certain extent upon the characteristics of the amplifier tubes. A milliammeter 43 in series relation with the resistor 31 provides an indication of the strength of the received signals which is of value when difficulty is experienced in obtaining proper operation of the printer P.

In accordance with a further feature of the invention, the gain control of the amplifier is given a time constant approximately equal to or faster than the maximum signal frequency but less than the carrier frequency or the frequency of the audible tone generated by the oscillator 03, thus substantially preventing bias of the signals impressed upon the printer P on the reception of strong signals over the line circuit and permitting adequate control on interrupted or discontinuous signals as compared with the continuous carrier in radio system. In order to secure the desiresd time constant, assuming a tone frequency of 750 cycles per second and a signaling frequency of 45.5 (half) cycles per second, condensers II and 42 of onetenth mf. capacity are connected in parallel relation to the resistors 35 and 36 respectively, giving the gain control a time constant of about one-fortieth of a second; i. e., the condenser ll or 42 is charged to approximately two-thirds of its final potential in one-fortieth of a second. It will-be apparent that for the assumed value of carrier frequency, the gain control having the time constant mentioned will not affect the carrier wave but will be operative to reduce the pe k f lar e nal impulses and cut off the tails of the signals. This feature is novel in gain control systems generally inasmuch as the ordinary gain control generally used in radio receivers is given a time constant considerably slower than the lowest audio component of the carrier in order to avoid distortion of the audio signals.

The operation of the gain control described above as shown in Fig. 1, does not entirely eliminate the bias of strong signals as will be apparent from the graph shown in Fig. 3E. The slight bias upon the occurrence of strong signals is substantially eliminated however, by the arrangement shown in Fig. 2. The manner '1 which this is accomplished will be apparent from Fig. 4 which is a comparison between the characteristics of the amplifiers shown in Figs. 1 and 2. Fig. 4A is similar to Figs. 33. and 3C and illustrate the operation of the gain control shown in Fig. 1 in response to weak and strong signals,

the first impulse representing a weak signal and .the other impulses representing strong signals.

The line a-z represents the bias upon the tube V3 which is varied in the manner described when the signals impressed upon the amplifier are of such magnitude that the' resistor R6 is traversed by grid current. Fig. 413 represents the corresponding signals impressed upon tubes V4 and V5, and it will be noted that the weaker signals have been amplified to a greater extent than the strong signals, and in the case of the stronger signals, the tails of the signals are amplified to a greater extent than the signals proper, resulting in a slight bias of the printer signals in the case of strong signals as shown in Fig. 4C. It will be understood that Fig. 4A and Fig. 4B are. not drawn to the same scale, the

scale being reduced in Fig. 4B. The line ai s! I in Fig. 43 represents the cut-013Ev bias on the tubes V4, V5.

Figs. 4D and 4E represent the operation of the gain control in the system shown in Fig. 2. In Fig. 4D the line 22-42 represents the bias on the rectifier tube V9, a portion of which is applied to the grid of tube W, as described above. The first signal impulse shown is a weak signal and the others abnormally strong signals. Assuming Fig. 4A represents the signals impressed upon the-tube V1, Fig. 2, in the case of strong signals the bias 22-22 of tube V9 increases as shown in Fig. 4D and thereby increases the bias on the tube V'I so that the. tails of the signals are not amplified to the same extent as in the amplifier of Fig. 1, as will be apparent by comj paring Fig. 43 with Fig. 4D. Therefore, as shown in Fig. 4E, thesignals in the output circuit of the amplifier are substantially unbiased in the case of strong signals as well as weak signals. It will also be apparent from Fig. 4 that since bias of the signals is also caused by disproportionate amplification of the tails of the signals, it is preferable to provide sufilcient grid bias on theinitial stage or stages of the amplifier so thatno grid current flows in the tube or tubes on strong signals. This is effected. for exampleby employing a resistor 38 of suclrresistance that the potential drop thereacross is quency currents, means for impressing onsaid channel signalling impulses ot definite predetermined duration each consis ing i a trai of electrical oscillations of audio frequency. a receiving recorder operated by said signalling impulses, means associated with said recorder for minimizing bias distortion of said impulses and means whereby said last mentioned means is controlled by the amplitude of the signalling impulses.

2. A printing telegraph system comprising a a telephone channel adapted to transmit voice frequency currents, means for impressing on said channel character code impulses of definite predetermined duration each consisting of a train of electrical oscillations of audio frequency, a receiving printer operated by said signailing impulses, an amplifier between said channel and said recorder and means associated with said amplifier responsive to each of said impulses for shaping the impulses impressed on said recorder according to the amplitude of the received impulses.

3. A printing telegraph system comprising a telephone channel adapted to transmit voice frequency currents, means for impressing on said channel character code impulses of definite predetermined duration each consisting of a train of electrical oscillations, a receiving printer operated by said signalling impulses, an amplifier between said channel and-said recorder and means for controlling the gain of said amplifier having a time constant substantially equal to the duration of each of said impulses for reducing bias .of the received signals.

4. A printing telegraph system comprising a telephone channel adapted to transmit voice frequency currents, means for impressing on said channel character code impulses of definite predetermined duration each consisting of a stant of the same order'as thatof the minimum signal impulse for compensating for distortion of the received signals.

6. A printing telegraph system comprising, means for transmitting code impulse signals consisting of an interrupted carrier current, receiving means responsive to said signals and a multistage amplifier associated with said receivingmeans; said amplifier embodying automatic gain control means for one stage of the amplifier connected to' be responsive to the signals impressed upon a later stage and having a time constant of the same orderas that oiothe shortest signal received.

1 '7. A printing telegraph system comprising means for transmitting code impulse signals consisting of an interrupted carrier current, receiving means responsive to said signals and a multistage amplifier associated withsaid receive ing means, said amplifier embodying automatic gain control means for one stage of the ampliv fier connected to 'be responsive to the signals impressed upon a later stage and having atime constant slower than the carrier frequency but faster than the maximum signal frequency.

8. A communication system comprising means for transmitting a carrier current interrupted in accordance with predetermined signals, receiving means responsive to said signals, amplifying means associated with said receiving means and having an input circuit, and means associated with the input circuit of said amplifier for shaping the signals impressed upon the receiving means, said last mentioned means including automatic gain control means having a time constant of the same order as that of the shortest signal received.

9. A communication system comprising means for transmitting a carrier current interrupted in .accordancewith predetermined signals, re-

ceiving means responsive to said signals, multistage amplifying and rectifying means associated with said receiving means and having an input circuit, and variable bias means associated with the input circuit of said amplifier and responsive to the signals impressed upon the rectifier stage for compensating for the effect of the inertia of the oscillating mechanical and electrical elements of the system upon the signals.

10. In a telegraph system utilizing an interrupted audible frequency carrier current, an amplifier for the telegraph signals having an input circuit, means in the input circuit for controlling the gain of the amplifier and means including a condenser connected to the input circuit of such capacity as to time, the gain control means to operate slower than the carrier frequency but faster than the signaling frequency.

11. In a telegraph system utilizing signalling impulses consisting of an interrupted carrier current, receiving means responsive to said impulses and a cascade or multistage amplifier for said impulses comprising a plurality of discharge tubes provided with grids and means for biasing said grids in accordance with the magnitude of the signalling impulses applied to the respective stages of the amplifier, said biasing means including a condenser arranged to retard the change in grid bias occurring at the end of each impulse, said condenser being so proportioned as to provide a time constant of the same order as the length of the shortest signaling impulse.

12. In a telegraph system utilizing signalling impulses consisting of an interrupted carrier current, receiving means responsive to said impulses, means including a grid-controlled rectifier tube for amplifying and rectifying the signals before the same are impressed upon the receiving means and means for controlling the gain of said amplifying means in accordance with the grid current of said rectifier tube, said gain control means embodying timing means to prevent operation thereof in response to the carrier current alternations but not to said impulses.

13. In a system of the character described. means for transmitting signalling impulses consisting of an interrupted carrier current, means for amplifying said impulses comprising a first amplifier tube, and a second amplifier tube in series relation with said first tube, means associated with said second amplifier tube for controlling the gain in both of said amplifier tubes, and a receiving device connected to said amplifier.

14. In a system of the character described, means for transmitting signalling impulses consisting of an interrupted carrier current, means for amplifying said impulses comprising 3 cascade amplifier embodying gain control having a time constant of the same order as the length of said impulses for the respective stages of the amplifier, means whereby said gain control operates in accordance with the magnitude of the signals at a stage later than the initial stage, and a receiving device connected to said amplifier.

15. In a system of the character described, means for transmitting signalling impulses consisting of an interrupted carrier current, means for amplifying said impulses comprising a cascade amplifier embodying gain control for the initial stages of the amplifier operating in accordance with the magnitude of the signals at a stage later than the initial stage, said gain control having a time constant slower than the carrier variation but faster than the maximum signal frequency, and a receiving device connected to said amplifier.

16. In a carrier-current impulse signalling system, an amplifier comprising an amplifier tube, a grid-controlled rectifier tube in series relation with said amplifier tube to rectify the signaling impulses and means for controlling the gain of both of said tubes in accordance with the grid current of said rectifier tube.

17. In a carrier-current impulse signaling system, an amplifier comprising a first tube provided with a grid and a second tube provided with a grid, means for controlling the gain of said tubes embodying resistors in series relation with the respective grids and arranged to be traversed by the grid current of said second tube upon the occurrence of strong signals, and condensers in shunt relation to said resistors of such magnitude as to impart a time constant to the gain control equal to or faster than the maximum signal frequency.

18. In a carrier-current impulse signaling system for use in conjunction with telephone systems, an amplifier comprising a first tube provided with. a grid biased so that no grid current flows on strong signals, a second tube in series relation with said first tube and provided with a grid and arranged to rectify the signals, and resistors connected to each other and to the respective grids and proportioned to control the grid bias of both tubes depending upon the magnitude of the signals impressed upon the second tube.

19. A communication system comprising a lowfrequency carrier channel including a source of carrier current, means for keying said carrier current to produce signaling impulses of definite predetermined duration, a receiving recorder operated by said signaling impulses, means associated with said recorder for minimizing distorticn or said impulses and means whereby said last mentioned means is controlled by the amplitude of the received impulses.

20. A communication system comprising a low-frequency carrier channel including a source of carrier current, means for keying said carrier current to produce character code impulses of definite predetermined duration, a receiving recorder operated by said impulses, a multistage amplifier between said channel and said recorder and means associated with said amplifier responsive to the amplitude of the received impulses for reducing the bias of the received signals, said last mentioned means being connected to separate stages of the amplifier in such a manner as to control one stage in accordance with the magnitude of the impulses impressed upon a later stage.

21. A printing telegraph system comprising a low-frequency carrier channel including a source of carrier current, means for keying said carrier current to produce character code impulses of definite predetermined duration, a receiving printer operated by said impulses and means associated with said printer for reducing the bias of the received signals, said means including an amplifier embodying automatic gain control means having a time constant of the same order as that of the shortest signaling impulses.

FERNAND E. DHUMY. LAWRENCE w. FRANKLIN. 

